1. Field of Application
The present invention relates to an electronic control apparatus such as an ECU (electronic control unit) used in a vehicle, that includes a plurality of microcomputers each equipped with a rewritable type of non-volatile memory, and in which each microcomputer can update data stored its non-volatile memory by writing externally supplied data into the memory.
2. Description of Related Art
Types of ECU are known, for controlling various equipment of a motor vehicle, which are made up of a plurality of microcomputers. Such a type of ECU is described for example in Japanese patent publication No. 9-160766.
Each microcomputer of such an ECU includes a rewritable type of non-volatile memory, for storing programs and control data. Data (i.e., constituting a program and/or control data) can be supplied to such a microcomputer, to be written into the non-volatile memory and thereby updating the memory contents. Generally the plurality of microcomputers are successively connected in series by internal communication lines, with one of the microcomputers also being connected to an external communication line, for transferring data and memory address information to a microcomputer of the ECU from an apparatus that constitutes a data source external to the ECU). For example, in the case of an ECU having two microcomputers, when data are to be supplied (for updating the contents of a non-volatile memory) from the external apparatus to the microcomputer which is not connected to the external communication line, then the microcomputer which is connected to the external communication line receives the data and forwards the data via an internal communication line to the other microcomputer.
In addition, as described for example in Japanese patent publication No. 7-069093, types of electronic control apparatus are known in which a first microcomputer that is connected to an external communication line (as defined above) and is connected to a second microcomputer via an internal communication line, converts signals that are received via the internal communication line from the second microcomputer into a protocol that is appropriate for an external apparatus, and transmits the converted signals via the external communication line to the external apparatus.
However in the case of an ECU such as is described in Japanese patent publication No. 9-160766 there is the following disadvantage. When data for memory updating are transmitted via an external communication line from an external apparatus, and the data are to be written into the non-volatile memory of a destination microcomputer which is not connected to the external communication line, it is necessary for that destination microcomputer to be capable of receiving data at the transmission rate of the external apparatus.
That is to say, with such a type of ECU, even if a microcomputer is connected only to one or more other microcomputers within the ECU, it is necessary for it to have the same high level of performance (i.e., data receiving processing performance) as a microcomputer that is connected to an external communication line and so receives data transmitted directly at a high data rate from an external (data source) apparatus.
Thus, although (if the communication protocol used by the destination microcomputer differs from that of the external apparatus) protocol conversion can be performed to enable data communication between the external apparatus and the destination microcomputer that is not connected to the external communication line, such communication will still be impossible if the data receiving processing performance of that destination microcomputer is insufficient. That is to say, the microcomputer which is connected to the external communication line will transfer the incoming data, from the external apparatus, to the destination microcomputer at the same rate as the data are transmitted from the external apparatus. Hence, if the data receiving processing performance of that destination microcomputer is insufficient, then overflow will occur in the transfer of data within the ECU.
For that reason, it has been necessary to use microcomputers which have the same high level of data receiving processing performance, both as a microcomputer that directly receives data from an external apparatus, and also as each of one or more other microcomputers which receive data only indirectly (i.e., by transfer within the ECU) from the external apparatus.
The terminal “receiving processing performance” is used here with the basic significance of “maximum rate at which data can be received by the microcomputer, without occurrence of data overflow”.
Hence it has been necessary for all of the microcomputers of such an ECU to be of an expensive type, even if transfer of data at a high rate (between microcomputers within the ECU) will only occur on the occasions when non-volatile memory contents are updated.